1. Field of the Invention
The present invention relates to a method of fabricating a diffraction grating, and more particularly to a method of fabricating an extremely fine diffraction grating on a base material by optical lithography employing near-field light.
2. Description of the Related Art
Semiconductor laser types in which a diffraction grating is formed on a region along an active layer to select and stabilize an oscillating frequency, such as distributed Bragg reflector (DBR) lasers, distributed feedback (DFB) lasers and the like, have hitherto been provided in a variety of ways. Recently, in this type of semiconductor layer, a demand for shortening of the oscillating wavelength is becoming increasingly strong from the standpoint of realizing a blue light source, etc. To meet this demand for a shorter wavelength, a technique for fabricating a diffraction grating with a micro-cycle, as well as selection of semiconductor materials, becomes indispensable.
If this diffraction grating is made into a higher-order diffraction grating, the cycle becomes relatively large and consequently the processing also becomes easy. However, in the higher-order diffraction grating there is a reduction in the feedback light quantity due to spatially diffracted light, and on top of that, there is a need to control the line-and-space ratio with a high degree of accuracy. Therefore, in many cases a first-order diffraction grating is preferred. In the case of a first-order diffraction grating, the linewidth of the grating pattern on the order of 100 nm or less is required.
In widely used methods for fabricating a fine diffraction grating, two-beam interference exposure has been applied to photolithography to form a fine diffraction grating pattern, as shown in Japanese Unexamined Patent Publication No. 6(1994)-148413, for example.
However, since in optical lithography the diffraction limit of light becomes the limit of resolution, it has been said that a linewidth of 100 nm (in terms of a diffraction grating cycle, 200 mm) is the limit of fining, even if an F2 eximer laser of wavelength 248 nm is employed. Furthermore, if resolution in the order of a nanometer greater than 100 nm is to be obtained, electron beam lithography or X-ray (particularly, synchrotron radiation) lithography must be employed.
The e-beam lithography has the following advantages:
basically a pattern in the order of a nanometer can be controlled and formed with a high degree of accuracy; the focal depth is considerably deep compared with optical systems; and direct wafer exposure (i.e., exposure of the resist directly by focused electron beam without a mask) is possible. However, the e-beam lithography has the drawback that it is still far from mass production, because the throughput is low and it requires expensive devices that cause an increased cost. This e-beam lithography also has the problem that it is difficult to maintain a uniform linewidth with respect to a wide area, as it employs scanning exposure.
The X-ray lithography is capable of enhancing resolution and precision by a factor of 10, compared with eximer laser exposure, even when 1:1 mask exposure is performed and even when a reflection-type image-forming X-ray optics system is employed. However, the X-ray lithography is difficult to realize because mask fabrication is difficult, and also has the problem that fabrication costs are high.
In the lithography employing an X-ray or e-beam, there is a need to develop specialized resists in accordance with the exposure methods, and there are still many problems associated with sensitivity, resolution, resistance to etching, etc.
The present invention has been made in view of the aforementioned problems. Accordingly, it is an object of the present invention to provide a method that is capable of inexpensively fabricating a diffraction grating with an extremely fine cycle. Another object of the invention is to provide a method which is capable of fabricating the aforementioned diffraction gratings in high throughput, while maintaining a uniform linewidth with respect to a wide area.
To achieve the aforementioned objects and in accordance with one aspect of the present invention, there is provided a diffraction-grating fabricating method comprising the steps of:
forming a photosensitive resist layer on a base material, either exposed or unexposed regions of the resist layer by emission of light becoming soluble in a developing solvent;
emitting near-field light to the resist layer by means for emitting near-field light according to a diffraction grating pattern upon reception of the light;
forming the diffraction grating pattern in the resist layer by developing the resist layer; and
forming a diffraction grating in the base material by etching the base material with the pattern in the resist layer as an etching mask.
In accordance with another aspect of the present invention, there is provided a diffraction-grating fabricating method comprising the steps of:
forming a first resist layer and a second resist layer on a base material in the recited order, the first resist layer being removable by etching and the second resist layer being a photosensitive resist layer in which regions either exposed or unexposed to emission of light become soluble in a developing solvent;
emitting near-field light to the second resist layer by means for emitting near-field light according to a diffraction grating pattern upon reception of the light;
forming the diffraction grating pattern in the second resist layer by developing the second resist layer;
etching the first resist layer with the pattern in the second resist layer as an etching mask and forming a diffraction grating pattern consisting of the first and second resist layers; and
forming a diffraction grating in the base material by etching the base material with the pattern in the first and second resist layers as an etching mask.
In a preferred form of the present invention, the second resist layer is formed to a thickness of 100 nm or less. On the other hand, the first resist layer is formed from non-photosensitive material and the step of etching the first resist layer is performed by dry etching.
In another preferred form of the present invention, the light has a wavelength of 250 to 450 nm and the diffraction grating has a cycle of 400 nm or less. Also, the diffraction grating is rectangular in cross section and the line-and-space ratio is between 1.2:1 and 1:1.2.
In still another preferred form of the present invention, the means for emitting near-field light is a mask, formed above a member permeable to the light, which emits the near-field light from a metal pattern having openings. In that case it is desirable that the light emission be performed with the metal pattern disposed in direct contact with the exposed resist layer formed on the base material, or in close proximity to the exposed resist layer in a range where the near-field light reaches the exposed resist layer. Also, the means for emitting near-field light may be an optical stamp formed from a member permeable to the light, a land-and-groove pattern being formed on one surface of the member and the optical stamp emitting near-field light according to the land-and-groove pattern when the light is guided through the inside of the member to the one surface and totally reflected at the one surface. In that case, it is also desirable that the light emission be performed with the optical stamp disposed in direct contact with the exposed resist layer formed on the base material, or in close proximity to the exposed resist layer in a range where the near-field light reaches the exposed resist layer.
In a further preferred form of the present invention, the means for emitting near-field light is a probe with an opening of smaller diameter than a wavelength of the light, and the light emission is performed by scanning the probe on the exposed resist layer formed on the base material.
The diffraction-grating fabricating method of the present invention mentioned above is extremely efficient when applied in fabricating a diffraction grating for a distributed feedback (DFB) laser or distributed Bragg reflector (DBR) laser.
According to the diffraction-grating fabricating method of the present invention, a diffraction grating of linewidth 100 nm or less, i.e., cycle 200 nm or less, which cannot be obtained by conventional photolithography, can be formed by exposing a resist layer to near-field light oozing out from a pattern of sufficiently smaller linewidth than the wavelength of light to be emitted and developing the exposed resist layer.
Hence, if such a diffraction-grating fabricating method is applied for fabricating a diffraction grating for a DFB or DBR laser, a DFB or DBR laser which oscillates in a short wavelength region less than a blue light region can be realized.
While resolution in conventional lithography is determined primarily by the wavelength of a light source, in the diffraction-grating fabricating method of the present invention the wavelength of the light source for emitting near-field light can be any wavelength. Therefore, there is no need to develop a novel light source and the fabricating costs for a diffraction grating can be considerably reduced.
In addition, in the aforementioned method employing the two-layer resist consisting of first and second resist layers, even in the case where there is a level difference in the base material and there are those portions of the first resist layer which near-field light does not reach, the second photosensitive resist layer on the first resist layer can be made uniform in thickness because a surface is planarized by the first resist layer. Hence, even a large-area pattern can be illuminated uniformly with near-field light and a minute pattern can be formed in the second resist layer. By patterning the first resist layer and the base material by a conventional etching method, with the pattern in the second resist layer as a mask, a fine pattern with a high aspect ratio can be easily formed at low cost.
Furthermore, in the case where the aforementioned mask with a metal pattern or the optical stamp with a land-and-groove pattern is employed as the means for emitting near-field light, a diffraction grating pattern with a large area can be exposed at once, unlike the case of scanning exposure and therefore diffraction gratings can be fabricated inexpensively and with high throughput.